1.Field of the Invention
The present invention relates to methods and means for test monitoring electrical power equipment--in particular, the insulating values of the dielectrics comprising means for confining the course of electric currents to prescribed conducting paths in and on high voltage ac electrical power equipment. By continuously, accurately, and rapidly monitoring such insulation power factor and/or capacitance, the instant invention provides for field testing, under a plethora of adverse conditions of external electrical stress and the like, for correctly indicating the true condition of same to determine whether or not it is safe to leave the equipment in service in the immediate future.
2. Description of the Prior Art
It has been noted that numerous prior art investigators have discovered, taught, and disclosed methods and/or means for determining the immediate condition of the insulating materials comprising various parts and configurations of ac electrical equipment, including that of the type used in applications of relatively high voltage. For example, the teachings of U.S. Pat. No. 1,945,263, Doble, Jan. 30, 1934, assigned to Doble Engineering Company relates to apparatus useful for measuring power factor. Doble's apparatus accurately measures insulation power factor and capacitance of electrical equipment, but is taught with the following limitations:
1. The equipment to be tested must be taken out of service and disconnected from associated equipment. PA1 2. The equipment is tested at a voltage that is much lower than its operating voltage. This is a consideration of considerable importance when it is realized that on some (particularly older) equipment, the insulation power factor is significantly higher when in service than can be measured by a low voltage test. PA1 3. The test can be conducted by at one point in time and must therefore be repeated periodically to establish trends in insulation quality. In many instances this is a formidable drawback since many times, it simply is not practical to test the insulation of a given piece of equipment often enough to detect deterioration thereof before it leads to rapid failure. PA1 4. Each test requires hands-on attention by an operator. PA1 5. Only one piece of equipment can be tested with any one test setup.
Other prior art investigators have taught and disclosed other methods and/or means for determining the immediate condition of such insulating materials. U.S. Pat. No. 3,710,242, Povey, Jan. 9, 1973, assigned to Doble Engineering Company discloses apparatus which effectively eliminates limitations (1) and (2) above. In a manner somewhat similar to the apparatus described and used in the present invention, Povey's apparatus measures power factor and capacitance by comparing a voltage derived from the capacitance tap with a reference potential. Unfortunately his advancement of the prior art does not eliminate limitations (3), (4), and (5) supra, and (as pointed out in Povey's disclosure) generally requires long, shielded leads to a voltage transformer (or other suitable reference with a known phase relationship) which may be located a long distance from the equipment being tested. For repeated tests, these leads must either be left in place indefinitely or rerun for each test.
The apparatus of the instant invention and the means and methods of employment of same effectively eliminates all five of the limitations mentioned above by providing automatic, continuous monitoring, with the equipment that is to be tested in service and energized at its operating voltage, and with minimal human interface. In addition, the practice of the present invention provides for apparatus which may be used to compute temperature corrections, averages, and short-term and long-term trends. It also gives alarms and can be connected to trip equipment out of the service under certain preset conditions. In contrast to the inventions of the type of the prior art, which test only one piece of equipment at a time, the present invention provides for apparatus which may be used to monitor any number of pieces of equipment simultaneously. As with Povey's equipment, leads must be run to a voltage transformer or other suitable reference, but only one (2-conductor, shielded) lead must be run for each phase of each voltage on which equipment is to be monitored, regardless of the number of pieces of equipment. A separate lead must be run from each piece of monitored equipment to the invention apparatus, which is usually located in the switchhouse or other central, sheltered location.
Of course, not all of the investigators of the prior art have directed their approaches to solving many of these problems heretofore associated with determining the state, or condition of, insulating materials normally associated with equipment of the type herein described to the determination of power factor and/or capacitance characteristics. One such other approach to advancing the state of this art is found in the teachings of U.S. Pat. No. 4,293,399, Belanger et al., Oct. 6, 1981, assigned to Hydro-Quebec wherein is described apparatus which is employed to monitor the hydrogen gas dissolved in insulating oil. This apparatus of Belanger may be employed as a continuous monitor, and thus eliminates the first four of the five limitations mentioned above for the original Doble apparatus. The Belanger et al. invention monitors hydrogen gas only on one piece of equipment; no other gasses are monitored and no electrical properties are monitored. The device provides alarms, but no hard data copies or trend analysis. An output is provided so the user can connect a strip-chart recorder if he so desires. Although the teaching of Belanger et al. have been reported to advance the state of this art it must be realized that, of course, the dissolved hydrogen monitor approach of this invention is not applicable to equipment that does not use oil in its insulation system.
Just which type of monitoring system (power factor/capacitance or dissolved hydrogen) provides the more desirable indication of insulation quality has been the subject of some debate. To this end, TVA has developed data which strongly indicates that power factor measurement provides a more reliable indication of insulation quality than hydrogen gas content, and usually indicates a destructive trend earlier, at least on some equipment which is not especially suited for gas monitoring. In any event, it should be noted that the apparatus described in the practice of the present invention can monitor (and compute averages and trends on) both power factor and hydrogen if the hydrogen monitor described above is installed and its output is wired into said apparatus.